Heretofore, as shown in FIG. 7, there has been known a vehicle seat device using a vehicle seat slide device 100. In the vehicle seat device, a vehicle seat frame 109 is assembled to the vehicle seat slide device 100. The vehicle seat frame 109 is provided with a seat cushion frame 110 supporting a seat cushion 90, a seatback frame 120 supporting a seatback 91, a recliner 140 which pivotably joins the seatback 91 with the seat cushion 90 for adjusting the angle of pivot movement, and a shoulder belt anchor 130 provided on an upper end of the seatback frame 120. Back side frames 121, 122 are arranged on both sides of the seatback frame 120, and a back center frame 123 taking the form of a hollow, square pillar is arranged at a center part of the seatback frame 120. Further, cushion side frames 111, 112 are arranged on the both sides of the seat cushion frame 110, and a cushion center frame 113 is arranged on a center part of the seat cushion frame 110. An upper arm 124 made of a thick plate is fastened by means of bolts to a lower port of the back center frame 123, and a lower arm 114 made of a thick plate is fastened by means of bolts to an upper part of the cushion center frame 113. Then, between the upper arm 124 and the lower arm 114, the recliner 140 is provided to pivotably join the upper arm 124 and the lower arm 114.
Further, a retractor 150 is attached to the seatback frame 120, and a seat belt 151 comprising a shoulder belt portion 151a and a lap belt portion 151b is arranged to have its one end wound up by the retractor 150. The other end of the seat belt 151 is secured to an anchor plate 115 attached to the cushion center frame 113, and the intermediate portion of the seat belt 151 passes through a tongue plate 152. The tongue plate 152 is able to be attached to, and detached from, a buckle 116 attached to the cushion side frame 111. The vehicle seat frame 109 is for two passengers, and description is omitted regarding a seat belt for a seat on one side.
As shown in FIG. 8, the vehicle seat slide device 100 is provided with a lower rail 101 fixed to a vehicle floor and an upper rail 102 supported movably relative to the lower rail 101. A horizontal upper portion of the upper rail 102 is joined with a horizontal lower portion of the cushion center frame 113 through a joining portion (e.g., a vehicle seat slide device described in Patent Document 1). Further, lower hook portions 104a, 104b each taking an inverted U-letter shape are formed at both ends of a lower hook member 104 secured to the lower rail 101, whereas upper hook portions 105a, 106a each taking a U-letter shape are formed at lower ends of upper hook members 105, 106 secured to the cushion center frame 113. The lower hook portions 104a, 104b and the upper hook portions 105a, 106a constitute a hook mechanism which absorbs a separation load acting on the upper rail 102 against the lower rail 101 (e.g., a vehicle seat slide device described in Patent Document 2).
In the vehicle seat device using the vehicle seat slide device 100, a moment acts to lift up the rear end of the cushion center frame 113 at the time of a vehicle collision (particularly, at the time of a frontal collision). As a result, a separation load against the lower rail 101 is exerted on the upper rail 102 through the joining portion between the cushion center frame 113 and the upper rail 102. Particularly, where the seat belt 151 is brought into lock, a large moment is exerted on the cushion center frame 113 through the shoulder belt anchor 130, the back center frame 123 and the like, whereby a heavy separation load against the lower rail 101 is exerted on the upper rail 102. However, it is deemed that even in such a case, the separation of the upper rail 102 from the lower rail 101 can be prevented in the vehicle seat slide device 100 because the lower hook portions 104a, 104b and the upper hook portions 105a, 106a are brought into engagements.    Patent Document 1: JP 2002-59770 A    Patent Document 2: JP 2002-87207 A